Water molecules in the intracellular or extracellular fluid are buzzing around at greater than 1000 mph

The molecular world is very very different than the world that we experience. So different that its just hard to really get it. At the molecular scale, lots of stuff happens spontaneously and randomly, we’ll get back to this. At the molecular scale, everything is in motion. At the level we experience the world, stuff just sits still unless we apply a force to it. My car just sits in my driveway. My water bottle just sits on my desk. We (famously incorrectly) think that we need to apply a force to an object to get it to move. The phospholipids in a plasma membrane are vibrating and shuffling around the surface of the cell. Inside the cell (or outside), the water molecules and dissolved solutes are buzzing around at speeds of about 1000 mph. Let’s focus on the water molecules. These are moving at an average speed of about 1300 mph. The molecules are in random motion. By random, I mean, given a “snapshot” of the water, there is no information in this “picture” that I can use to predict the speed and direction of individual water molecules. Random does not mean “uncaused”. An individual molecule is moving the speed and direction that it’s moving because of its own unique history, which includes importantly, the molecule that it just bumped into and bounced off of. The moving water (and other) molecules have a form of energy called kinetic energy (KE). The kinetic energy of a moving object is \(KE = \frac{1}{2}mv^2\) where \(m\) is the molecule’s mass and \(v\) is the molecule’s velocity (if we ignore direction then \(v\) is the speed). KE is an important concept to hang onto for understanding human physology.

Water has a number of nearly magical properties that are both useful, and cause problems, in physiology. All of these properties occur because of the special chemistry of a water molecule.

Water is a polarized molecule because of the polar covalent bonds formed between the hydrogen atoms and the highly electronegative oxygen atom. When hydrogen forms a covalent bond with oxygen, the single electron that should be buzzing around each hydrogen nucleus is attracted to the oxygen nucleus so has a higher probability of buzzing around the oxygen then the hydrogen. This makes the oxygen side of the molecule electron dense and the hydrogen side of the molecule electron poor, on average. Consequently there is a high desnity of negative charge on the oxygen side and a low density on the hydrogen side. And because positively charged stuff is attracted to negatively charged stuff and negatively charged stuff is attracted to positively charged stuff, water molecules are extremely attracted to

each other – the hydrogen side of one molecule is attracted to the oxygen side of a second molecule. This attraction creates a kind of chemical bond called a hydrogen bond.
molecules that are charged, or have regions of high or low electron density (because of polar covalent bonds or because of loss or gain of protons or electrons). A molecule that interacts with water in this way is hydrophillic. A molecule that doesn’t is hydrophobic.

These interactions are very short lived (tiny fractions of a second) and very weak. They are short lived because the bond is week and the KE of the moving water molecules and other molecules is high enough to disrupt the interaction, or break the bond.

Let’s dig a little deeper into the chemistry of a polarized covalent bond

An atom is the fundamental unit of matter. It is composed of the subatomic particles protons and neutrons in the nucleus at the center of the atom and electrons that buzz around the periphery. A proton has a single positive charge. An electron has a single negative charge. The number of protons in the nucleus defines the element, add a proton and you have a new element. Hydrogen has a single proton, Carbon has 6 protons, Oxygen has 8 protons.
A neutrally charged atom has the same number of electrons as protons.
One useful model for thinking about electrons is that they are arranged in layers around the nucleus. There is a limited number of electrons that can occur in these layers. For example the innermost layer can contain at most 2 electrons, the next layer can contain at most 8. If the outermost layer has the maximum number of electrons than it is “full”.

An atom with a full outermost layer is reactively inert, meaning that it doesn’t react with other atoms. The elements with full outermost layers are the elements in the rightmost column of the periodic table. All other elements are reactively unstable – they don’t hang around long as an isolated atom but instead react with other atoms to form atomic bonds – an electrical attraction between atoms. The bond creates a sort of stability where both atoms in the bond have filled (or something like filled) outermost layers. The reaction might create a new molecule, or a dissolved ion that binds to water, or simply bind the atom to another molecule.
In an ionic bond, one atom gives up the electrons in its outmost layer while the other atom takes these to fill its outermost layer. An example is NaCl – table salt. The Na has a single electron in its outer layer while the Cl is one short have having a full outer layer. The two atoms form an ionic bond where the Na gives up its outermost electron to the Cl. Now both have full outermost layers (for the Na, this is the next layer closer to the nucleus).
In a covalent bond, outermost electrons are shared between the atoms. By share, you have to think probabalistically. The electron has some probability of being in the outermost layer of one atom and another probability of being in the outermost layer of the other atom. Remember - electrons aren’t sitting still. They are buzzing around.

Some elements are more attractive to shared electrons so the electrons in the covalent bond are shared “unequally”. This property is electronegativity. Oxygen is very electronegative. In covalent bonds between oxygen and hydrogen (which is very common in our molecules), the shared electrons have a much higher probability of buzzing around oxygen’s outermost layer than hydrogen’s outmost layer. On average, the oxygen side of the bond is more negative because there are more electrons than protons, while the Hydrogen side is more positive because there are fewer electrons than protons. This is called a polar covalent bond because the bond is polarized. “Polarized” means something changes along an axis. A polarized congress is one with a very sharp divide with everyone with one opinion on one side and the opposite oppinion on the other.
Consider a water molecule, or \(\mathrm{H}_2\mathrm{O}\), which is an oxygen atom that forms two polar covalent bonds, each with a hydrogen atom. There is a high probability that at any instant, a hydrogen’s single electron will be in the oxygen’s outermost layer. As a consequence, the distribution of charges is asymmetric – the oxygen side of the water molecule has the 8 protons of the oxygen atom bond but tends to have 10 electrons, the 8 of the oxygen and one each from the hydrogens. So the oxygen side is negatively charged. And, the hydrogen side has the 2 protons (one for each hydrogen atom) but 0 electrons, because the single electron tends to hang out with the oxygen, so the hydrogen side is positively charged. On the whole the water molecule is electrically neutral – there is no net charge – there are 10 total protons and 10 total electrons. But the distribution of charges is asymmetric and one side of the water molecule is negatively charged and the other side is positively charged. Water is a highly polarized molecule. This is what gives water all of its seemingly magical properties.
Now consider a fatty acid. Most of the bonds are between carbon and hydrogen and these are covalent bonds with roughly equal sharing of the outermost electrons. As a consequence, the electrons are pretty evenly distributed around the molecule and everywhere there is the same amount of negative charge from the electrons and positive charge from the protons.

What is “dissolved”?

The consequence of #2 is that water makes a really good solvent. Stuff dissolves in water. When a chemical dissolves in water, it doesn’t simply get dispersed uniformly around but becomes part of the structure of the water. By “structure” I mean these short-lived bonds. If we take a snap shot of dissolved glucose in water, we “see” not glucose or water but a new molecule: water bound to glucose (and water bound to water). If we take a snapshot a fraction of a second later, we see the glucose bound to different water molecules. So this concept of structure is very dynamic. Any substance that interacts with water in this way is hydrophillic. Hydrophilc molecules are soluble in water. That is, they dissolve. When the substance is dissolved, it is a solute. And water is the solvent. Together, the water and solutes form a solution.

Some particles (molecules or larger solids) don’t interact with water–they are hydrophobic. If we stir up the water, we can get these molecules to disperse fairly uniformly throughout, but this isn’t dissolved. This is called a colloid if the particles are the size of molecules and a suspension if the particles are bigger (than about one micrometer). Milk is a colloid – its full of triacylglycerols that do not readily dissolve in water.

“Hydrophilic” and “hydrophobic” is dichotomaniac thinking. In reality, some molecules are more soluble and some molecules are less soluble in water. In general, some combination of small, polar, or charged molecules are the most soluble and large, non-polar, or non-charged are the least soluble. Molecular oxygen (O2) and CO2 are pretty soluble because they are small – water isn’t really that attracted to them but they are simply small enough to be, at least ephemerally, trapped between water molecules that are attracted to each other. Proteins can be pretty large molecules, but proteins are pretty soluble (at least generally, we will talk about insoluble proteins) because they tend to have lots of polar covalent bonds and charged regions because of acidic or alkaline amino acids.

Most fatty acids and other lipids are not very soluble, although some very small fatty acids are pretty soluble.

Conception/Misconception – hydrophobic has the roots hydro = water and phobic = fearing. A reasonable interpretation of this is that water repels hydrophobic molecules. A better way to think about it is, water molecules are just really attracted to each other. So, as all the molecules are jiggling around, the water molecules tend to re-arrange or re-organize in a way that maximizes water-water interactions. This occurs when hydrophobic molecules are all aggregated together into a single fat drop. The system will just naturally move toward this state (this is an example of entropy).

It takes a lot of energy to evaporate water

Humans have evolved to exploit this as a cooling mechanism called evaporative cooling. Temperature is a measure of the thermal energy of a system. In a system of water molecules moving around, the higher the kinetic energy the higher the thermal energy, the higher the temperature. In water, the H2O molecules are moving around at different speeds and, consequently, have a distribution of kinetic energies. The higher the KE the less likely colliding H2O molecules will interact with a hydrogen bond. If the KE of a H2O is high enough, it cannot interact with other H2O molecules and enters a gas phase – it evaporates. Now consider a human, which is 99% water molecules. If a human is hanging out in Death Valley, where heat flows into the body increasing the KE of the H2O molecules, or a human is exercising heavily, where heat lost from metabolic reactions increases the KE of the H2O molecules, then the distribution of KE of the H2O shifts to the right (see my drawing) and some of the molecules acquire enough energy to evaporate. Since the H2O that evaporates are at the high end of the KE distribution, the molecules that are left in the human have a lower, mean KE than before the evaporation. As a consequence the human is cooler, because the mean KE is lower.

water makes a great skeletal element, if you can keep it from flowing

I’ll explain more when we talk about the skeleton. But here is a preview:

water has high surface tension, which is awesome if you are an insect that walks on the surface of a lake, but not so good if you are a human and require inflated alveoli

I’ll explain more when we talk about the respiratory system, but here is a preview

Water explainer